Metal Oxide Catalysts Applied for Carbon Dioxide Electroreduction Reaction

Abstract

Due to a recent rapid increase of the CO2 concentration in the Earth’s atmosphere, there is a worldwide necessity to search for suitable and capable catalytic systems for CO2 conversion. Since electrochemical reduction of CO2 (CO2RR) is a promising approach, the work presented in this dissertation deal with the greenhouse gas CO2 by using metal oxide catalysts to convert it to value-added fuels. The main statement is to produce electrocatalysts which can be applied for CO2 electroreduction. In Part I, we prepared hexagonal and monoclinic phases of La2O2CO3 nanoparticles by different wet preparation methods and investigated their phase-related CO2 behavior through field-emission scanning microscopy, high-resolution transmission electron microscopy, Fourier transform infrared, thermogravimetric analysis, CO2-temperature programmed desorption, and linear sweeping voltammetry of CO2 electrochemical reduction. The monoclinic La2O2CO3 phase was synthesized by a conventional precipitation method via La(OH)CO3 when the precipitation time was longer than 12 h. In contrast, the hydrothermal method produced only the hexagonal La2O2CO3 phase, irrespective of the hydrothermal reaction time. The La(OH)3 phase was determined to be the initial phase in both preparation methods. During the precipitation, the La(OH)3 phase was transformed into La(OH)CO3 owing to the continuous supply of CO2 from air whereas the hydrothermal method of a closed system crystallized only the La(OH)3 phase. Based on the CO2-temperature programmed desorption and thermogravimetric analysis, the hexagonal La2O2CO3 nanoparticles (HL-12h) showed a higher surface CO2 adsorption and thermal stability than those of the monoclinic La2O2CO3 (PL-12h). The crystalline structures of both La2O2CO3 phases predicted by the density functional theory calculation explained the difference in the CO9 behavior on each phase. Consequently, HL-12h showed a higher current density and a more positive onset potential than PL-12h in CO2 electrochemical reduction. In Part II, ZnO /La2O2CO3 composite materials have been used as a catalyst for CO2 electroreduction reactions and the morphology of macropores an important factor to influence on catalyst performance. In this study, we prepared La2O2CO3/ZnO composite materials by two different methods – precipitation (CoLZ) and ethylene glycol combustion (LZ) – as functions of La/Zn ratios and calcination temperatures. All of the materials prepared by the solution combustion method clearly showed disordered macroporous morphology whose framework was composed of ZnO and La2O2CO3 nanoparticles. The addition of ZnO can promote the production of CO, and the dominant products of ZnO/La2O2CO3 are CO. The CO2RR onset potential of CoLZ was more negative than those of the LZ, and LZ showed the lower interfacial charge transfer resistance. Moreover, the LZ sample can form C2H4 products by further converting the CO. In Part III, Cu/La2O2CO3 composite materials have been used as a catalyst for CO2 electroreduction reactions and the loading of copper can cause phase conversion, which play an important role in the catalyst performance. In this study, we prepared La2O2CO3 support by ethylene glycol combustion (EL) and the copper was doped through hydrothermal method-as functions of Cu/La2O2CO3 ratios. The materials prepared clearly showed copper lattice grew into disordered macroporous framework. EL-Cu-x series catalysts were well prepared by hydrothermal method and the main phase of copper was converted from CuO to Cu2(OH) 3Cl with the increasing content of copper precursor. The unique nanostructures cuprous chloride derived Cu can suppress the HER and enhance the CO2RR catalytic performance. For further improving the electrical conductivity and surface area of the catalysts, the four kinds of carbon supports were employed. Except the carbon spheres were synthesized by hydrothermal method, the other three carbon supports including carbon nanotube, graphene oxide and activated carbon were commercially purchased. And it was found the largest surface area and mesoporous AC-Cu sample showed the highest CO2RR catalytic activity (FEC2H4=50.8%) which is keep consistent with its lowest interfacial electron transfer resistance.